1. Field of the Invention
The invention relates to a computer tomography method which involves helical scanning of an examination zone by means of a scanning unit which includes a radiation source and a detector unit, an object present in the examination zone and the scanning unit simultaneously rotating about an axis of rotation relative to one another and performing a motion parallel to the direction of the axis of rotation, resulting in a relative motion in the form of a helix, and also involves a reconstruction of the spatial distribution of the absorption within the examination zone from the measuring data acquired by the detector unit. The invention also relates to a computer tomography apparatus for performing a method of this kind.
2. Description of the Related Art
A method and a computer tomography apparatus of the kind set forth are known from DE-OS 195 45 778 (Tam). This known method enables the scanning of an extended examination zone in the direction of the axis of rotation by means of a cone beam and the reconstruction of the absorption distribution in the examination zone also in case the object present therein, for example a patient, is longer than the part of the examination zone for which data has been acquired.
However, for this method it is necessary to define in advance the so-called region of interest (ROI) and at the beginning and at the end of the ROI (in relation to the direction of rotation) an additional scan of the examination zone must be performed along a circular path which extends perpendicularly to the axis of rotation. The reconstruction of the absorption distribution may commence only after the scanning of the examination zone has been completed. The change over from a circular scan to a helical scan and back to a circular scan of the examination zone necessitates an abrupt acceleration or deceleration of either the scanning unit or the object in the examination zone; this could cause unsharpness. It is a further drawback that the scanning region must be defined in advance.